1. Field of the Invention
The present invention relates to a closed caption decoder for extracting and decoding a transmitted caption signal multiplexed on a video signal thereby to produce caption information, and a television receiver having the closed caption decoder for displaying the caption information together with an image on the screen.
2. Description of Related Art
A closed caption system fox transmitting the same information as a voice in the form of caption signal multiplexed on the scanning signal during the vertical retrace period of the video signal has been employed for many television programs, video software, etc., in order to enable deaf-and-dumb persons and persons having a difficulty in hearing to enjoy the television broadcasting to the same degree as persons having no difficulty in hearing do. The encoding and transmission of the caption information are specified in detail by FCC (Federal Communication Commission) standards. Also, the closed caption system is used as a means for aiding in the understanding English by persons whose vernacular language is not English.
FIG. 1 is a block diagram showing a conventional closed caption decoder for extracting and decoding a caption signal from a video signal to produce caption information. FIG. 2(a) is a diagram showing a waveform of a video signal multiplexed with a caption signal, and FIG. 2(b) a waveform of a caption signal. The caption signal as caption information is multiplexed on the 21st H scanning signal, for example during the vertical retrace period.
A data slicer 1 extracts the multiplexed caption signal from the video signal and applies it to a decoder 2. The decoder 2 analyzes the display mode (caption, text), display style (paint-on, pop-on, roll-up), contents of characters, types of characters (display color, italic), display position, etc. of the extracted caption signal (caption information), according to the standards, and applies the result of analysis to a screen display controller 3. The screen display controller 3 supplied with the video signal, in accordance with the analysis result of the decoder 2, controls the character type and display position and produces a video signal containing the caption information, so that the title (caption information) as shown in FIG. 3 is displayed on the screen of television receiver together with the image.
FIGS. 4(a) and 4(b) are diagrams for explaining the display address of the caption information on the screen. The display of the caption information is required for securing a view without any information drop-out on all the household television receivers. This region called a safe caption area is designated as shown in FIG. 4(a). In FIG. 4(a), characters A and B designate the height and width respectively of the television image. In FIG. 4(a), characters C, D, E and F designate the height, width, vertical position and horizontal position respectively of the safe caption area.
The safe caption area covers equally 15 rows and 32 columns for a total of 480 character frames as shown in FIG. 4(b) in order to permit character display at accurate positions. The display addresses are allotted to each of the 480 character frames, and are designated by the movement of the cursor (hypothetical mark invisible on the screen) based on the combination of a preamble address code (PAC) and a tab offset code (TOC) which are determined at the sending side of the video signal. PAC is a code composed of the row number and one of eight "numerals for identifying the character frame (indent)". Each indent is designated by moving the cursor from the left end rightward by four columns. For example, indent 0 designates column 1, and indent 1 column 5. TOC has three control codes (TOC1, TOC2 and TOC3) including rightward cursor movement by one column, two columns and three columns, respectively. As a result, a row is designated by PAC and a column also by PAC in a unit of four columns, followed by movement of the cursor by 1, 2 or 3 columns by TOC, thereby making it possible to designate the address of all the character frames.
The caption information is displayed by this address designation in the character frame at an arbitrary position in the safe caption area as intended at the sending side.
In the conventional closed caption decoder configured as described above, the display position of the caption information is uniquely determined by address designation at the sending side. In the case where the display position of the character according to closed caption is designated at the lower area of the screen, it may be overlapped on the superimposed information (subtitle, etc.) in the picture. If the character according to closed caption is displayed on the credit line of the particular broadcast station, the names of the casters or performers, the news title or the like on display during a news or show program, for example, these superimposed information are hidden undesirably. For this reason, in the case where the caption decoding functions, although the voice information can be understood by the character display according to closed caption, the superimposed information inherently incorporated in the video information is hidden, thereby leading to the problem of a lower degree of understanding.